1. Field of the Invention
The present invention relates to semiconductor circuits, and particularly to a semiconductor circuit having a filter function.
2. Description of the Background Art
There are conventional semiconductor circuits that have a noise filter function and a signal delay function. Such a conventional semiconductor circuit includes a plurality of inverter circuits connected in series and a capacitor connected between ground and a node between given ones of the inverter circuits.
Now, a time constant is determined by adjusting a capacitor and the inverter circuit preceding the capacitor.
In this conventional semiconductor circuit, the time constant is determined so that, at the node between the given inverter circuits, the signal rising inclination and the signal falling inclination differ from each other. For example, the time constant is determined so that the signal rises slowly and falls sharply.
Then a filter function is obtained by utilizing the difference between the signal rising inclination and the falling inclination and the threshold of the following inverter.
While related prior arts include the invention disclosed by Japanese Patent Application Laid-Open No. 7-95022 (1995: FIG. 1), the object of the invention described therein is not to obtain a filter function. That is to say, when a plurality of small-width noise pulses are successively superimposed on the input signal, it is difficult to properly filter off the noise pulses.
The conventional semiconductor circuit mentioned above is capable of filtering off only one of L-level noise superimposed during originally H signal periods and H-level noise superimposed during originally L signal periods.